Structured Query Language
SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems.
SQL statements are used to retrieve and update data in a database.
SQL works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, Sybase, etc.
Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).
Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!
SQL is a Standard – BUT...
SQL Database Tables
A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders").
Tables contain records (rows) with data.
Below is an example of a table called "Persons":
Last Name First Name Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
With SQL, we can query a database and have a result set returned. A query like this:
SELECT LastName FROM Persons
Gives a result set like this:
SQL Queries
Last NameLast Name
Hansen
Svendson
Pettersen
Data Manipulation Language (DML)
SQL (Structured Query Language) is a syntax for executing queries. But the SQL language also includes a syntax to update, insert, and delete records.
These query and update commands together form the Data Manipulation Language (DML) part of SQL:
•SELECT - extracts data from a database table •UPDATE - updates data in a database table •DELETE - deletes data from a database table •INSERT INTO - inserts new data into a database table
SQL Data Definition Language (DDL)
The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted.
We can also define indexes (keys), specify links between tables, and impose constraints between database tables.
The most important DDL statements in SQL are:
•CREATE TABLE - creates a new database table •ALTER TABLE - alters (changes) a database table
•DROP TABLE - deletes a database table
•CREATE INDEX - creates an index (search key)
•DROP INDEX - deletes an index
SQL The SELECT Statement
The SELECT statement is used to select data from a table.
The tabular result is stored in a result table (called the result-set).
Syntax
SELECT column_name(s)FROM table_name
To select the columns named "LastName" and "FirstName", use a SELECT statement like this:
SELECT LastName, FirstName FROM Persons
Last Name First Name Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
Persons
Last Name First Name
Hansen Ola
Svendson Tove
Pettersen Kari
Vysledok
To select all columns from the "Persons" table, use a * symbol instead of column names, like this:
SELECT * FROM Persons
Select All Columns
Last Name First Name Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
Persons
The result from a SQL query is stored in a result-set. Most database software systems allow navigation of the result set with programming functions, like: Move-To-First-Record, Get-Record- Content, Move-To-Next-Record, etc.
Programming functions like these are not a part of this tutorial. To learn about accessing data with function calls, please visit our ADO tutorial.
The Result Set
Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.
Some SQL tutorials end each SQL statement with a semicolon. Is this necessary? We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each SQL statement, but some database programs force you to use it.
Semicolon after SQL Statements?
The DISTINCT keyword is used to return only distinct (different) values.
The SELECT statement returns information from table columns. But what if we only want to select distinct elements?
With SQL, all we need to do is to add a DISTINCT keyword to the SELECT statement:
Syntax
SELECT DISTINCT column_name(s)
FROM table_name
The SELECT DISTINCT Statement
To select ALL values from the column named "Company" we use a SELECT statement like this:
SELECT Company FROM Orders
Using the DISTINCT keyword
Company Order Number
Sega 3412
W3Schools 2312
Trio 4678
W3Schools 6798
Order
Company
Sega
W3Schools
Trio
W3Schools
Note that "W3Schools" is listed twice in the result-set.
To select only DIFFERENT values from the column named "Company" we use a SELECT DISTINCT statement like this:
SELECT DISTINCT Company FROM Orders
Order
Company Order Number
Sega 3412
W3Schools 2312
Trio 4678
W3Schools 6798
Company
Sega
W3Schools
Trio
Select All Columns
The WHERE clause is used to specify a selection criterion.
The WHERE Clause
To conditionally select data from a table, a WHERE clause can be added to the SELECT statement.
Syntax
SELECT column FROM table
WHERE column operator value
With the WHERE clause, the following operators can be used:
Note: In some versions of SQL the <> operator may be written as !=
Operator Description
= Equal
<> Not equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
BETWEEN Between an inclusive range
LIKE Search for a pattern
Using the WHERE Clause
To select only the persons living in the city "Sandnes", we add a WHERE clause to the SELECT statement:
SELECT * FROM PersonsWHERE City='Sandnes'
Last Name First Name Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978
Svendson Stale Kaivn 18 Sandnes 1980
Pettersen Kari Storgt 20 Stavanger 1960
Last Name First Name Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978
Svendson Stale Kaivn 18 Sandnes 1980
Using Quotes
Note that we have used single quotes around the conditional values in the examples.
SQL uses single quotes around text values (most database systems will also accept double quotes). Numeric values should not be enclosed in quotes.
For text values:
This is correct:
SELECT * FROM Persons WHERE FirstName='Tove'
This is wrong:
SELECT * FROM Persons WHERE FirstName=Tove
The LIKE Condition
The LIKE condition is used to specify a search for a pattern in a column.
Syntax
SELECT column FROM table
WHERE column LIKE pattern
A "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern.
Using LIKE
The following SQL statement will return persons with first names that start with an 'O':
SELECT * FROM Persons
WHERE FirstName LIKE 'O%'
The following SQL statement will return persons with first names that end with an 'a':
SELECT * FROM Persons
WHERE FirstName LIKE '%a'
Using LIKE 2
The following SQL statement will return persons with first names that contain the pattern 'la':
SELECT * FROM Persons
WHERE FirstName LIKE '%la%'
The INSERT INTO statement
The INSERT INTO statement is used to insert new rows into a table.
Syntax
INSERT INTO table_name
VALUES (value1, value2,....)
You can also specify the columns for which you want to insert data:
INSERT INTO table_name (column1, column2,...)
VALUES (value1, value2,....)
Insert a New Row
And this SQL statement:
INSERT INTO Persons
VALUES ('Hetland', 'Camilla', 'Hagabakka 24', 'Sandnes')
Last Name First Name Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
Last Name First Name Address City
Pettersen Kari Storgt 20 Stavanger
Insert Data in Specified
Columns
And This SQL statement:
INSERT INTO Persons (LastName, Address)
VALUES ('Rasmussen', 'Storgt 67')
Last Name First Name Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
Rasmussen Storgt 67
Last Name First Name Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
The Update Statement
The UPDATE statement is used to modify the data in a table.
Syntax
UPDATE table_name
SET column_name = new_value
WHERE column_name = some_value
Update one Column in a Row
We want to add a first name to the person with a last name of "Rasmussen":
UPDATE Person SET FirstName = 'Nina'WHERE LastName = 'Rasmussen'
Last Name First Name Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmuseen Nina Storgt 67
Last Name First Name Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmuseen Storgt 67
Update several Columns in a Row
We want to change the address and add the name of the city:
UPDATE Person
SET Address = 'Stien 12', City = 'Stavanger'
WHERE LastName = 'Rasmussen' Last Name First Name Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmuseen Nina Stien 12 Stavanger
Last Name First Name Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmuseen Storgt 67
The Delete Statement
The DELETE statement is used to delete rows in a table.
Syntax
DELETE FROM table_name
WHERE column_name = some_value
Delete a Row
"Nina Rasmussen" is going to be deleted:
DELETE FROM Person WHERE LastName = 'Rasmussen'
Last Name First Name Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmuseen Nina Stien 12 Stavanger
Last Name First Name Address City
Nilsen Fred Kirkegt 56 Stavanger